Lithographic techniques for photoresist patterning with resolutions down to the 22 nm node are essential for future device miniaturization. (See: The 2003 Edition of the ITRS: Lithography, 2005 International Technology Roadmap for Semiconductors: http://public.itrs.net/2003). Patterning at the nanometer scale can be achieved in various ways including optical, atomic force microscope, scanning probe, electron beam, nanoimprint, and extreme ultraviolet lithography (EUV lithography). In order to keep pace with the demand for rapid printing of smaller features, it is necessary to gradually reduce the wavelength of light used for imaging and to design imaging systems with larger numerical apertures.
EUV lithography, for example, uses short wavelength (13.4 nm) radiation to administer projection imaging or lithographic patterning. As a result of its short wavelength radiation, EUV lithography has evolved into a possible candidate for the production of future integrated circuits at the 45 or 22 nm mode. Much of the work to date, however, has focused on aspects of lithography tool development as opposed to resist performance.
Two primary types of resist polymers that have been investigated are chain-scission resists and chemically amplified (CA) resists. Upon irradiation of a chain-scission resist film, the molecular weights of the polymers in the exposed regions are decreased via chain scission reactions arising from the irradiation. As a result, solubility differentiation is achieved between the exposed and the unexposed regions. Chemically amplified resists achieve solubility differentiation based on an acid-catalyzed deprotection reaction which changes the polarity of the polymer in the exposed region. A typical CA resist formula consists of a matrix polymer and a photoacid generator (PAG). Upon irradiation with an electron beam or extreme UV radiation, the PAG generates a strong acid that catalyzes the deprotection reaction.
Several classes of PAGs have been used in CA resists. These PAGs, however, are almost exclusively used in their small molecule forms, and small molecule PAGs typically exhibit limited compatibility with the resist polymer matrix. As a result, problems such as phase separation, non-uniform acid distribution, and non-uniform acid migration occurring during temperature fluctuations (e.g. variation in baking processing) may arise. Such limitations frequently lead to an undesirable, premature and non-uniform deprotection reaction in the CA resist film.
Moreover, resists for EUV lithography and other lithographic techniques must possess reasonable photospeed while maintaining a low level of outgassing components. Lithographic resists must additionally demonstrate high sensitivity, high contrast and resolution, low absorption, high dry-etch resistance, good adhesion to substrates, and low line-edge roughness. Current resists for sub-100 nm patterning applications, including EUV lithography, display poor etch resistance, poor outgassing properties, and undesirable absorption coefficients. As a result, it would be desirable to provide PAGs and resists that exhibit improved properties for lithographic processes such as EUV, X-ray (XRL), 193 nm, electron beam (EBL), and ion beam (IBL) lightographies.